This invention relates to a semiconductor circuit on a large scale integrated circuit (LSI). In particular, it relates to a complementary source follower circuit of a MOS (metal oxide semiconductor) LSI that is suitable for an output buffer of an analog circuit.
FIG. 1 shows a conventional CMOS (complementary MOS) drain follower circuit. In FIG. 1, there are two transistors, a P-channel MOSFET (metal oxide semiconductor field effect transistor) and an N-channel MOSFET. Each transistor has a source (S), a drain (D), a gate (G), and a body (B). This circuit can be used to invert signals, i.e., to convert a logical “0” into a logical “1” and a logical “1” into a logical “0,” where a logical “0” is represented by the voltage Vss and a logical “1” is represented by the voltage Vcc. The logical “1” or “0” signal is supplied to “IN” and the inverted logical “0” or “1” signal is sent to “OUT.” The voltages Vss and Vcc are kept constant during the operation of the circuit. Each of the transistors should be “off” when the other transistor is “on” but there is some overlap when both transistors will be “on,” resulting in a “DC path” with a current flow, Id, from Vcc to Vss. This current flow is wasted power.
FIG. 2 illustrates the relationship between the voltage at IN in FIG. 1 (Vin along the horizontal axis) and the voltage at OUT in FIG. 1 (Vout along the right vertical axis) and shows that as Vin approaches the voltage Vcc, Vout approaches the voltage Vss and as Vin approaches the voltage Vss, Vout approaches the voltage Vcc. The left vertical axis gives the current Id (dotted curve) that flows as Vin increases along the horizontal axis from Vss to Vcc and shows that when Vin is about midway between Vss and Vcc there is a current flow Id, which is lost power.
The circuit of FIG. 1 is useful for processing digital signals because digital signals consist of a logical 0 or a logical 1, and intermediate levels where power is lost occur only during transitions between them. However, as circuits shrink, it is becoming possible to perform both digital and analog functions on a single chip, which is especially desirable for wireless and mobile applications.
FIG. 3 shows an example of an analog signal. This signal consists of many intermediate voltage levels. When an analog signal is inputted to conventional CMOS circuit, such as that of FIG. 1, the current Id flows almost constantly because the voltage is at intermediate levels so much of the time.
FIG. 4 shows a complementary source follower circuit similar to the complementary drain follower circuit of FIG. 1, where the sources, rather than the drains, are connected to the “OUT” voltage. FIG. 5 is similar to FIG. 2 but shows (horizontal axis and right vertical axis) that as the INPUT voltage in FIG. 4 increases from Vss to Vcc the OUTPUT voltage Vout increases from Vss to Vcc. FIG. 5 also shows (horizontal axis and left vertical axis) that there is a small current loss Id (dotted curve) at a voltage intermediate between Vss and Vcc. The level of current Id depends on the threshold voltage conditions. When the voltage applied to the transistors does not exceed their threshold voltages, both transistors are in an off condition and there is no “Id” current flow. For that reason, source follower circuits are attractive for merged digital-analog systems.
FIG. 6 shows a source follower circuit having an active load. But in the circuit of FIG. 6, when the transistor is “on” a current steadily flows between Vcc and Vss, which increases power consumption. Also, the driving power is low because the current is shared between both the active load and the output load.
In order to reduce the power consumption and the driving power loss, a complementary source follower circuit can be used as shown in FIG. 7. This circuit can be realized using a bulk semiconductor substrate such as a single crystal silicon N-type or P-type substrate, but for a P-type substrate the source-to-body connection of the N-channel transistor is eliminated (as in FIG. 7) and for an N-type substrate the source-to-body connection of the P-channel transistor is eliminated (not shown).
However, eliminating the source-to-body connection in FIG. 7 results in non-linearity between the output voltages of the two transistors because the characteristics of the two transistors are not symmetrical.
In U.S. Pat. No. 5,463,240, a complementary source follower circuit on a common substrate is achieved by isolating each P-channel and N-channel transistor from the common semiconductor substrate. In laid open Japanese Patent No. 2000-323720 by the instant inventor, an SOI (silicon on insulator) substrate is used and in that invention no additional mask or circuit is needed, because on an SOI substrate each transistor can be isolated from other transistors and from the substrate.
As shown in FIG. 8, the linear relationship between input voltage Vin and output voltage Vout shown in FIG. 5 is offset when both transistors are “off” at the same INPUT voltage. Non-linearity occurs because the INPUT voltage is lower than the threshold voltages of the transistors. This creates a “dead gap” at an intermediate voltage between Vss and Vcc where both transistors are “off” and the voltage at the OUTPUT node is open. This dead gap can be tolerated when the signal is digital, but it distorts an analog signal. Improvements in the linearity between the input signal and the output signal are needed in order to permit a source follower circuit to process analog signals.
The circuit shown in FIG. 9 is similar to the circuit shown in FIG. 4 except that a voltage VthN is added to the input voltage Vin going to the N-channel MOSFET and a voltage VthP is subtracted from the input voltage Vin going to the P-channel MOSFET. As shown in FIG. 10, these shifts in the two input voltages shift the voltage output Vout to the dotted line, thereby eliminating the dead gap.
In U.S. Pat. No. 6,333,623, a source follower circuit is used as a voltage regulator because a source follower circuit has low output impedance. In this patent, level shift circuits are also disclosed. In this patent, a level shift circuit is applied to the input node (IN) of the complementary source follower circuit to solve the dead gap problem. Also, a source follower circuit is used as a voltage regulator because a source follower circuit has low output impedance. In this patent, level shift circuits are also disclosed.
In the above patents, in order to realize complementary source follower circuits additional manufacturing processes are needed and, in order to solve dead gap problem, additional circuits, such as a level shift circuit, are needed.